Byung-Chang Kang, Soon-Bo Lee, Jin-Hyo Boo

Transcription

1 Ž. Surface and Coatings Technology Growth of TiO thin films on Siž 100/ substrates using single molecular precursors by metal organic chemical vapor deposition Byung-Chang Kang, Soon-Bo Lee, Jin-Hyo Boo Department of Chemistry, Sungkyunkwan Uni ersity, Suwon , South Korea Abstract Growth of titanium dioxide Ž TiO. thin films on SiŽ 100. substrates was carried out using a single molecular precursor at deposition temperature in the range of C by the metal organic chemical vapor deposition Ž MOCVD. method. TitaniumŽ IV. isopropoxide, ŽTi OCHŽ CH.. 3 4, was used as a precursor without any carrier gas. Crack-free, anatase type TiO polycrystalline thin films with a stoichiometric ratio of Ti and O were successfully deposited on SiŽ 100. at temperature as low as 500 C. XRD and TED data showed the formation of the highly oriented anatase phase with the 11 direction for the TiO thin films grown on SiŽ 100. at below 500 C, whereas with increasing the deposition temperature to 700 C, the main film growth direction was changed to be 00, suggesting a possibility of epitaxial thin film growth. Two distinct growth behaviors were observed from the Arrhenius plots. Below 500 C, the growth rate of TiO is apparently limited the substrate temperature. The activation energy for TiO film deposition calculated in this region is approximately 77.9 kj mol, while that for a film grown above 500 C shows a negative value, indicating a predominant diffusion controlled deposition process. Using Al TiO p-si metal-insulator semiconductor Ž MIS. diode structure, a dielectric constant was also obtained from a capacitance-voltage Ž C V. curve to be Elsevier Science B.V. All rights reserved. Keywords: TiO film; Anatase phase; Single molecular precursor; Metal organic chemical vapor deposition; Two activation barriers 1. Introduction Titanium oxide Ž TiO. thin films have valuable properties such as high refractive index 1, excellent transmittance in the visible and near-ir frequency, and high chemical stability. Therefore, they are extensively used in anti-reflection coating, sensors and photocatalysis for electrical and optical applications 3. Specially, TiO has a high dielectric constant of 180 along the c axis and 90 along the a axis 4, so it is useful in fabricating dielectric capacitors in micro electronic devices 5. Moreover, oxides of titanium are the Corresponding author. Ž. address: jhboo@chem.skku.ac.kr J. Boo. basis of many ternary perovskite materials such as BaTiO 3, SrTiO 3, and PbTiO 3. These materials are well known to have many potential applications in microelectronics 6. The bulk TiO materials are known to have three different kinds of polymorphous crystalline forms: rutile, anatase, and brookite 7. Among them, the TiO exists mostly as rutile and anatase phases, and both of phases are tetragonal structures. The rutile phase is always formed at higher temperatures, while the anatase phase is formed at lower temperatures and transformed into rutile phase above 800 C 8. Various deposition techniques have been developed for depositing TiO thin films, including evaporation, sputtering 9, thermal oxidation of titanium and the chemical vapor deposition Ž CVD. method 10. Among them, the $ - see front matter 000 Elsevier Science B.V. All rights reserved. Ž. PII: S

2 CVD technique using metal-organic compound as a precursor Ž MOCVD. has many advantages, such as good conformal coverage, the possibility of epitaxial growth and selective deposition and the application to large area deposition. Also, this method is low cost and it is easy to control the deposition growth parameters. Thus the MOCVD method is well known as one of the most powerful techniques and is suitable for stoichiometric and micro-structural thin film deposition 11. In this paper, we report the growth and characterization of anatase phase TiO thin films deposited on Si Ž 100. substrates at temperatures in the range of C, and we also discuss the influence of the deposition temperatures on the growth rate properties of TiO films. B. Kang et al. Surface and Coatings Technology Experimental Film depositions were performed using a homemade MOCVD system. A MOCVD apparatus was fabricated using a quartz tube and stainless steel bodies connected through O-ring joints. The SiŽ 100. substrate was pretreated with acetone, ethanol, de-ionized water, and HF solution in an ultrasonic cleaner, and mounted onto the graphite susceptor, which was laid in the center of MOCVD chamber. To fix the SiŽ 100. substrate onto the graphite susceptor tightly, we grooved the graphite susceptor and tilted the susceptor at an angle to get a thin film with a uniform surface. The MOCVD apparatus was evacuated using a mechanical pump. The graphite susceptor was heated using a DC power through Super-Kanthal wire inserted into it, and substrate temperatures were measured by a Chromel- Alumel Ž K-type. thermocouple. After maintaining substrate temperature at 00 C, we introduced the precursor into the chamber and raised gradually the substrate temperature to the deposition temperature. The deposition temperature was maintained in the range of C. TitaniumŽ IV. isopropoxide ŽTi OCH- Ž CH. ; TIP, 97%. 3 4, was used as a single molecular precursor. Since TIP is very volatile at low temperature and contamination from TIP is small 1, it was not necessary to use any carrier or reactive gas to increase mass transportation or to remove the contaminants in the film. The base pressure of the MOCVD apparatus was torr, and the working pressure was kept in the range of torr. The as-grown films were characterized with X-ray photoelectron spectrometry Ž XPS., X-ray diffraction Ž XRD., Rutherford backscattering spectroscopy Ž RBS., transmission electron microscopy Ž TEM. transmission electron diffraction Ž TED. and scanning electron microscopy Ž SEM.. Fig. 1. XPS survey and high resolution spectra of the TiO thin film grown on SiŽ 100. at 350 C Ž. 1 without sputtering and Ž. with 3 min. Ar ion sputtering: Ž. a XPS survey spectra; Ž b,c. XPS high resolution spectra of O1s and Tip peaks, respectively. 3. Results and discussion Fig. 1a shows the typical XPS survey spectrum of the as-deposited TiO thin film grown on SiŽ 100. substrate at a deposition temperature of 350 C. It shows the strong XPS peaks of Ti p and O1s as well as C 1s. The C1s peak at binding energy of 85.0 ev is attributed to the sample before Ar ion bombardment. After Ar ion bombardment of approximately 3 min, however, the area of the C1s peak decreases, and other peaks are shown more clearly. The presence of this peak is originally related to surface contamination, since the sample is exposed to air before the XPS analysis. Figs 1b,c shows the O1s and Ti p high-resolution XPS spectra obtained: Ž. 1 before and Ž. after an Ar ion bombardment, respectively. In Fig. 1b, two oxide states attributed to O and OH species were observed from the as-deposited TiO thin film without Ar ion bombardment. However, the OH shoulder peak disappeared after ion bombardment and the main O peak Ž ev. 1s shifted its binding energy to the reference value of O peak 13. This indicated that the OH peak is just a surface contamination peak

3 90 B. Kang et al. Surface and Coatings Technology probably coming from H O in the air. However, the Ti p3 peak showed a single peak at binding energy of ev before ion bombardment, which was at- 4 tributed to Ti 13. After Ar ion bombardment, the Ti p3 peak showed a shoulder on the low binding energy side, which was evidenced by the presence of 3 Ti 14, which explained that O could be removed from the surface by preferential sputtering. The atomic composition of Ti and O of the thin film was nearly 1:, which is good agreement with the stoichiometry value of the bulk TiO. In the case of the other thin films deposited at different temperature conditions, the same results were obtained within the error limit of XPS. Fig. shows the XRD patterns of deposited films on SiŽ 100. substrates under different temperature conditions. All XRD patterns display characteristic peaks of anatase TiO at 48.0 and 55.0 that are attributed to diffraction of anatase TiO Ž 00. and Ž 11., respectively. This is in good agreement with results reported previously 15. In addition to these two strong peaks, some weak peaks of 38.0 and 63.0, which were attributed to the anatase TiO Ž 004. and Ž 04. were also observed in the case of the films grown at below 500 C. Peaks of rutile and brookite phases were not observed. With increasing deposition temperatures, the XRD patterns show a tendency to change the main peak from Ž 11. to Ž 00. phase. At the temperature range of C, the films have a preferential Ž 11. orientation. At temperatures higher than 500 C, the Ž 11. orientation disappears rapidly, whereas the Ž 00. orientation appears gradually between 500 and 700 C. At deposition temperature as high as 700 C, the highly oriented TiO thin films in the 00 direction were obtained. These XRD results indicate that the singlephase polycrystalline TiO thin films with anatase structure could be deposited on SiŽ 100. substrates at deposition temperatures as low as 700 C, and the deposition temperatures can influence the film quality and growth direction. According to the plan-view SEM images Ž not shown., we concluded that the surface morphologies of TiO thin films are generally quite uniform and crack-free, but some roughness surfaces with defects were observed from the films grown at the low deposition temperatures. From the cross-sectional SEM images Ž not shown. the changes of film thickness was obtained as a function of deposition temperatures. In the temperature range of C, the thickness of deposited films increases from 1.0 m for 350 C to 14 m for 500 C with increasing temperatures. At the higher temperatures between C, however, thickness of films is decreased to approximately 3 m. This means that XRD and SEM data showed that the growth rate and film quality of TiO thin films were affected by the deposition temperatures. RBS was also applied to estimate the composition ratio of the thin film and thickness of the deposited Fig.. XRD patterns of the TiO films grown on SiŽ 100. substrates at different deposition temperatures. films. Fig. 3a shows a RBS spectrum obtained from a TiO thin film grown on SiŽ 100. at 350 C. First, we observed that the thickness of TiO film layer is approximately 1.03 m, i.e. quite a close value with the thickness obtained from cross-sectional SEM. Second, three peaks were observed corresponding to the Si substrate, O, and Ti, respectively. The stoichiometry of the film was calculated using RUMP simulation, resulting in good stoichiometry between Ti and O. This result is in good agreement with XPS composition analysis. The dependence of the film growth rate on the deposition temperatures was also clearly observed in this study. The film growth rate was obtained by the changes of film thickness that was measured by crosssectional SEM, RBS and alpha-step profiling analysis. The dependence of growth rate on substrate temperature is shown in Fig. 3b. Two growth temperature regions are apparent. When the deposition temperatures are between 300 C and 500 C, the film growth rate increases exponentially with substrate temperature according to the Arrhenius equation. This can be explained by the growth rate of TiO thin film being limited by the substrate temperature. This behavior is characteristic of a deposition process known as kinetically controlled deposition in which the surface decomposition of the precursor is the rate determining step 16,17. The activation energy for the TiO film deposition using TIP calculated from the slope of the Fig. 3b

4 is approximately 77.9 kj mol. In the higher temperature region over 500 C, the Arrhenius plot shows a positive slope, indicating that the growth rate is controlled by the mass transport of reagents through the boundary layer to the growth surface, resulting in no dependency of growth rate on the surface temperature. This is termed the region of diffusion controlled growth, and there may possibly be occurring some desorption and or pre-reaction with CO or residual gases in the MOCVD apparatus 16,17. This phenomenon shows the same tendency as XRD and SEM results for which the turn-over points of the variation of film thickness, crystal size and film growth direction are divided by two distinct regions at near 500 C. Fig. 4a shows TEM and TED patterns of a TiO film deposited at 450 C. Electron diffraction pattern mainly displays a mixture image of rings and weak spots, indicating a polycrystalline structure. The arising spot pattern indicates that our deposited film has some preferred orientations. Regarding our XRD data, the brightest spots might be attributed from TiO Ž 11. and Ž 00. diffraction, and the ring patterns are caused by TiO Ž 004. and Ž 04. phases. The interface image of B. Kang et al. Surface and Coatings Technology Fig. 4. Ž. a Cross-sectional TEM image of the TiO thin film grown on SiŽ 100. substrate at 450 C. The insert shows the TED pattern of the same film; Ž b. Capacitance voltage Ž C V. curve for the TiO thin film deposited on p-siž 100. at 450 C. Fig. 3. Ž. a RBS analysis of the TiO thin film grown on SiŽ 100. substrate at 350 C; Ž b. dependence of TiO film growth rate on the deposition temperatures. TiO Si shows very sharp and columnar structure, indicating good adhesion and a homogeneous TiO film layers at depth, but we could see some twin structures with a size of approximately 30 nm Ž see Fig. 4a.. The high frequency Ž 1 MHz. C V characteristic of Al TiO film p-si MIS diode structures was measured at room temperature to determine the dielectric constant of a TiO thin films. Fig. 4b shows the C V curve for a TiO film deposited at 450 C. A hysteresis loop was observed in the C V curve as shown in Fig. 4b. The clockwise type of hysteresis loop results from mobile charge, which is due primarily to the ionic impurities 18,19. Surface accumulation occurs for a

5 9 B. Kang et al. Surface and Coatings Technology negative bias greater than 4 V, and we obtained a dielectric constant of approximately 1 for the TiO thin film. The arising of a hump near the 0.5 V can results from the surface roughness with many defect sites in the thin film, and there is a slight decay in the inversion voltages above approximately 0.5 V, indicating a current leakage in the TiO film 18, Conclusions The growth and characterization of TiO thin films were carried out on SiŽ 100. substrates by MOCVD method at temperatures in the range of C. Highly oriented crack-free, stoichiometric anatase phase TiO polycrystalline thin films with good adhesion were successfully deposited on SiŽ 100. at temperature as low as 500 C. The deposition temperatures are influenced by the film quality and growth direction of TiO thin film. The temperature dependence of the growth rate and film quality was also studied. Two distinct growth behaviors were observed, and their activation energies were calculated from the slopes of the Arrhenius plots. Below 500 C, the growth rate of TiO is apparently limited by the substrate temperature. The experimentally obtained activation energy for TiO film deposition is approximately 77.9 kj mol, while that for a film grown above 500 C shows a negative value, indicating a predominant diffusion controlled deposition process. XRD and SEM show the same behavior as growth rate change near 500 C, signifying that the film growth direction changes above 500 C. From C V measurements of the Al TiO p-si structure, the dielectric constant was calculated as approximately 1 for the TiO thin film. Acknowledgements Support of this research by the Korea Research Foundation through the interdisciplinary program Ž Grant No. D and by the Korea Science and Engineering Foundation through Grant No is gratefully acknowledged. References 1 G.S. Brady, H.R. Clauser, Materials Handbook, thirteenth ed, McGraw-Hill, New York, 1991, p I.H.K. Pulker, Appl. Opt. 18 Ž C. Martinet, V. Paillard, A. Gagnaire, J. Joseph, J. Non-Cryst. Solids 16 Ž A. Erbil, W. Braun, B.S. Kwak, J. Crystal Growth 14 Ž T. Won, S. Yoon, H. Kim, J. Electrochem. Soc. 139 Ž S.A. Cambell, D.C. Gilmer, X.-C. Wang et al., IEEE Trans. Electron. Devices 44 Ž F.A. Grant, Rev. Mod. Phys. 31 Ž R.J.G. Clark, The Chemistry of Titanium and Vanadium, Elsevier, New York, 1968, p K. Yoshimura, T. Mike, S. Tanemura, J. Vac. Sci. Technol. A 5 Ž Q. Zhang, G.L. Griffin, Thin Solid Films 63 Ž E.K. Kim, M.H. Son, S.-K. Min, Y.K. Han, S.S. Yom, J. Cryst. Growth 170 Ž L.M. Willams, D.W. Hess, J. Vac. Sci. Technol. A 1 Ž J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, 1995, pp P. Babeleon, A.S. Dequiedt, H. Mosefa-Sba, S. Bourgeois, P. Sibillot, M. Sacilotti, Thin Solid Films 3 Ž T.W. Kim, M. Jung, H.J. Kim, T.H. Park, Appl. Phys. Lett. 11 Ž A.C. Jones, P. O Brien, CVD of Compound Semiconductors, sixth ed, VCH Verlagsgesellschaft mbh, 1997, pp A.C. Jones, T.J. Leedham, P.J. Wright et al., J. Materials Chem. 8 Ž.Ž D.K. Schroder, Semiconductor Material and Device Characterization, Wiley-Interscience, New York, 1990, pp K.S. Yeung, Y.W. Lam, Thin Solid Films 109 Ž